1. Field of the Invention
The invention relates to a method for mapping an original image from a source gamut color subspace to a destination gamut color subspace belonging to a certain output device and having a certain black point, the method comprising a step of black point compensation. The invention also relates to a system for mapping an original image from a source gamut color subspace to a destination gamut color subspace belonging to a certain output device and having a certain black point comprising a filter for decomposing the original image in several bands, yielding at least a low pass image and a high pass image; and a black point compensation component. The invention further relates to a computer program for performing the method and a computer readable medium for carrying the program.
2. Description of Background Art
Gamut mapping algorithms consider an input device's color gamut and an output device's color gamut. Some gamut mapping algorithms are said to be adaptive when they also consider some of the image characteristics, such as the image type, histogram, contrast or gamut. Recent techniques consider the image layout and the spatial features. They keep track of the spatial features and try to preserve them in the rendering process. In the ICC workflow, the color conversion algorithm consults the ICC profiles of the two devices (the source device and destination device) and the user's rendering intent (or intent) in order to perform the conversion. Although ICC profiles specify how to convert the lightest level of white from the source device to the destination device, the profiles do not specify how black should be converted. The user observes the effect of this missing functionality in ICC profiles when a detailed black or dark space in an image is transformed into an undifferentiated black or dark space in the converted image. The detail in dark regions (called the shadow section) of the image can be lost in standard color conversion.
Adobe Systems implemented Black Point Compensation to address this conversion problem by adjusting for differences between the darkest level of black achievable on one device and the darkest level of black achievable on another.
Black Point Compensation (BPC) also referred to as linear XYZ scaling maps the source's black point to the destination's black point in the CIEXYZ color space, hence scaling intermediate color values.
Alternatively, a Lightness Compression Algorithm (LCA) also named lightness scaling, resealing or remapping might be applied to the image in the CIELAB color space. Linear, polynomial and sigmoidal LCAs have been proposed in the prior art and have been implemented in point-wise (i.e. non spatial) color workflows. Experimental results suggest that the performance of sigmoidal scaling depends on the magnitude of gamut difference and might be image-dependent. XYZ scaling is considered as a baseline color re-rendering for reasonably similar output-referred source and destination media. Most point-wise ICC workflow implementations apply linear CIEXYZ scaling (e.g. Adobe).
Lightness scaling is also proposed in existing spatial gamut mapping algorithms, e.g. a linear compression to the low spatial-frequency band in the log domain, or also an optional sigmoidal lowest spatial-frequency band. Similar techniques have also been used to render High Dynamic Range (HDR) images, where the range of the base layer is compressed using a scale factor in the log domain of the rgb pixel values.
In prior art systems it is known that a user makes the choice to select BPC or not in converting an image. BPC can be considered as a gamut compression algorithm. As such, it produces images that are less saturated. This desaturation is not always welcomed and/or necessary. The effect is dependent of the actual content of the image and of the gamut of the destination device. This implies that, each time the user wants to make a print out of an image he has to decide whether the conversion of a particular image looks better with or without BPC. The user is only able to judge this after having seen a print out of the image, since BPC is device dependent. Even a preview on a screen will not help him. This is cumbersome.